There has been a need for some years to transfer multilayer (ML) technology, which has proven itself and has been known for a long time in the manufacture of ceramic multilayer capacitors, to other ceramic components as well. The transfer of a first modification of the ML technology onto varistors is known, for example, from European Patent 0 189 087, incorporated herein. The varistor is thereby constructed of thin layers of varistor material having precious metal electrodes lying therebetween, these being respectively conducted out at an end side and being connected to one another with a metallization (solder area). The precious metal electrodes that have a relatively high melting point are applied onto the thin ceramic layers with silk screening and before the sintering process in this modification of the ML technology.
The transfer of ML technology onto temperature-dependent thermistors has hitherto been disclosed, not for thermistors having a negative temperature coefficient (NTC, high-temperature conductors), but only for PTC elements (posistors) and only within the framework of a second modification of the known ML technology (U.S. Pat. No. 4,766,409, incorporated herein). In this modification, the ceramic member is alternately constructed of porous and dense ceramic layers, whereby metal alloys whose melting temperatures are considerably lower than the sintering temperature of the ceramic member are pressed into the cavities of the porous intermediate layers. The internal electrodes are thus produced after the sintering process by being pressed in and by subsequent solidification of the molten metal, whereby the penetration of the molten metal, the moistening of the ceramic material, and preventing the molten metal from flowing out again, raise a number of problems that, for example, are described in German Published Application 37 25 455, incorporated herein.
U.S. Pat. No. 4,766,409 initially proceeds on the basis that ML technology is especially suitable for the realization of a PTC thermistor having a resistance of only about 0.3 through 3 ohms on the basis of the parallel connection of many thin ceramic layers within a single component. Attempts to manufacture such a PTC thermistor are disclosed in the Letters Patent, a refractory metal paste being applied onto the ceramic layers before the sintering by analogy to the most widespread ML ceramic capacitors. The metals having a high melting point that come into consideration (gold, platinum, palladium, silver-palladium alloy), however, did not lead to functioning internal electrodes since, according to the Letters Patent, barrier layers arose. It has in fact been known for a long time that complications involving non-conductive barrier layers at the ceramic surface metallized with precious metals can arise in PTC resistors, but not in NTC thermistors.
Due to the test results, the U.S. patent states that these refractory metals are unsuitable for internal electrodes of PTC resistors. The U.S. Letters Patent subsequently points out that electrodes free of barrier layers which are composed of an indium-gallium alloy as well as of nickel or aluminum are known, but, as its own solution, proposes that internal electrodes of lead, tin, or of an alloy of these two metals, be pressed into porous intermediate layers of ceramic after the sintering. Such internal electrodes are in fact free of barrier layers. The metals employed, however, have poor moistening properties, for which reason additional protective measures to prevent the injected, molten metals from flowing out must be undertaken, these making the known PTC resistor even more complicated.